1. Field of the Invention
The present invention relates to control of liquid flows on a microfluidic chip and in particular to routing, reacting and mixing liquid microstructures on the surface of a rigid or flexible substrate using a combination of one or more of the following: high resolution temperature control at individual addressable electronic elements for generating thermocapillary flow of the liquid; surface patterning for confining the liquid at one or more particular locations on the chip; obtaining heterogeneous surfaces with hydrophilic and hydrophobic regions; and retaining hydrophilicity of surfaces.
2. Description of the Related Art
Technological developments in the miniaturization and integration of multiple functionalities for chemical analysis and synthesis into a hand held device have generated interest in developing efficient methods for transporting ultra small volumes of liquid through networked arrays. Conventional techniques include micromechanical and electric field driven methods for manipulating and controlling flow including pneumatic actuation, electro-osmotic, electrophoretic or electrowetting techniques, centrifugation and magnetic field driven pumping.
U.S. Pat. No. 6,124,138 describes a device for detecting or quantitating one or more of a plurality of different analytes in a liquid sample. The device includes a sample distribution network having a sample inlet, one or more detection chambers and a dead end fluid connection between the chambers and the inlet. A liquid sample is applied to the sample inlet and is drawn by vacuum action to deliver the sample to the detection chambers. The delivered sample reacts with at least one analyte specific reagent in each detection chamber under conditions effective to produce a detectable signal. A temperature controller can be used to heat or cool the detection chambers to facilitate reaction of the sample with the analyte detection reagents.
U.S. Pat. No. 6,306,273 describes a method for transporting a liquid in a channel of a microfluidic system by electrokinetic action in which a liquid containing material comprises a plurality of charged chemical species. A voltage is applied from one point along the channel to a different point along the channel whereby the charged chemical species are transported along the channel. Fields in the range of 200–600 V/cm are typically used to control the electro-osmotic flow.
One developed thermally based device uses thermocapillary pumping for pushing discrete liquid droplets through an enclosed microfabricated channel, as described in T. S. Sammarco and M. A. Burns, Thermocapillary Pumping of Discrete Drops in Microfabricated analysis devices [AIChE Journal45:350–366 (1999)]. This device allows discrete liquid plugs to advance through an interior channel by locally heating or cooling one of the droplet endcaps which induces a differential in capillary pressure. Micromechanical devices have the limitation that moving parts, such as miniature pumps and gears, often suffer leakage and degradation under wear. Electrokinetic and pneumatic techniques have the limitation that because the flow is confined to interior channels, particulates and aggregates in solution can block flow and destroy pumping action. Electrically based methods typically require external supply voltages of several kV. Each of the above-described devices also has the limitation of being capable of handling either continuous streams or discrete droplets but not both due to the flow mechanism on which they are based.
The mixing of liquids in microscale devices is often difficult to attain due to the absence of turbulent phenomena. Many devices therefore rely on the action of diffusion and laminar flow. It is therefore desirable to provide a method and system for routing, reacting and mixing small volumes of liquid on the surface of a substrate, without the need for moving parts, high pressure or vacuum. It would also be advantageous to operate such devices using low voltages which are compatible with conventional integrated circuits.